The present invention generally relates to a resonant DC-DC converter and particularly to a resonance DC-DC converter in which AC power is applied to a resonant transformer from a suitable AC power source and an AC output power of the transformer is rectified is supplied to a given load.
Such a resonant DC-DC converter is disclosed, for example, in U.S. Pat. No. 4,504,895. FIG. 8 is a diagram showing the resonant DC-DC converter disclosed in U.S. Pat. No. 4,504,895 but somewhat summarized for the sake of explanation. In FIG. 8, the DC-DC converter comprises: a DC power source 1; an inverter 2 for receiving DC power from the DC source and for converting the DC power into AC power, the inverter having a first series connection of first and second transistors Tr.sub.1 and Tr.sub.2 respectively acting as a first and a second switching element respectively connected to a positive and a negative pole of the DC power source 1, a second series connection connected in parallel to the first series connection and composed of third and fourth transistors Tr.sub.3 and Tr.sub.4 respectively acting as third and fourth switching elements disposed so as to respectively correspond to the first and second transistors, and first, second, third and fourth diodes D.sub.1 -D.sub.4 anti-parallelly connected to the first, second, third and fourth transistors Tr.sub.1 -Tr.sub.4 respetively; a transformer 3 connected to an output of the inverter for boosting an output voltage of the inverter; a rectifier 4 for converting an AC output voltage of the transformer into a DC output voltage; electrostatic capacitance C for smoothing an output voltage from the rectifier 4, and a load 5 connected to an output of the rectifier 4. The transistors Tr.sub.1 -Tr.sub.4 are arranged to be driven by a frequency determination circuit 6 and a frequency control circuit 7 through driving circuits 8a to 8d, respectively.
The transformer 3 is used to isolate the input and output of the converter from each other, and used to boost or reduce the output voltage in the case where the input voltage is different from the output voltage. Particularly, in the case where a high voltage of several tens KV to 200 KV is generated, for example, in a power source for generating X-rays, the turn ratio of the transformer 3 is very large and hence the number of turns of the secondary winding is very large. Accordingly, the secondary windings are formed in layers which are stacked one on one while being insulated one from one with insulators such as insulating sheets interposed between adjacent layers. As a result, stray capacitances C.sub.S1 -C.sub.Sn are formed between the layers of the secondary windings in the transformer 3 as shown in FIG. 9A. The circuit of FIG. 9A can be expressed by such an equivalent circuit as shown in FIG. 9B. That is, the serial capacitances C.sub.S1 -C.sub.Sn form a stray capacitance C.sub.S of the secondary winding. Further, the transformer 3 per se may be expressed by leakage inductance L.sub.1 and L.sub.2 and excitation inductance L.sub.ex and therefore the whole of the transformer 3 may be expressed by those inductance L.sub.1, L.sub.2 and L.sub.ex together with the stray capacitance C.sub.S as shown in FIG. 9C. Further, generally, L.sub.1 &lt;&lt;L.sub.ex and L.sub.2 &lt;&lt;L.sub.ex, so that leakage inductance L.sub.S being parasitic on the transformer 3 is expressed by L.sub.S =L.sub.1 +L.sub.2 and the equivalent cirucit of the transformer 3 can be shown as FIG. 9D.
If such a transformer 3 is used, the leakage inductance L.sub.S being parastic on the transformer and the stray capacitance C.sub.S of the secondary winding can be used as resonance elements so that a voltage induced at the stray capacitance C.sub.S by the resonance between the leakage inductance L.sub.S and the stray capacitance C.sub.S and the transformation ratio of the transformer 3 is supplied to the rectifier 4 and the output voltage of the rectifier 4 is applied to the load 5 after being smoothed through the electrostatic capacitance C. In order to control the output power to be supplied to the load 5, the ratio F.sub.i /F.sub.o of the operation fequency F.sub.i to the resonance frequency F.sub.o determined by the leakage inductance L.sub.S and the stray capacitacne C.sub.S is varied by the frequency determination circuit 6 and the frequency control circuit 8 shown in FIG. 8. That is, in the graph of FIG. 10, the respective curve shows the relation between the ratio F.sub.i /F.sub.o and the output voltage V.sub.o of the transformer 3 with the ordinate and abscissa representing the input voltage from the DC power source 1 and the output voltage V.sub.o and with load resistance R.sub.1, R.sub.2, . . . , R.sub.5 (R.sub.1 &gt;R.sub.2 &gt;. . . &gt;R.sub.5) as parameters. Since the resonance frequency takes a constant value determined by the leakage inductance L.sub.S and the stray capacitance C.sub.S of the transformer 3, the output voltage V.sub.o is controlled by suitably varying the operation frequency F.sub.i of the inverter 2.
In the thus arranged conventional resonant DC-DC converter, as seen in FIG. 10, the output voltage V.sub.o becomes maximum when the value of the ratio F.sub.i /F.sub.o is about "1", and if the operation frequency F.sub.i of the inverter 2 is made lower or higher than the resonance frequency F.sub.o, the output voltage V.sub.o is reduced. However, the output voltage V.sub.o cannot be reduced to zero. In this case, the output voltage V.sub.o can be made to approach zero if the operation frequency F.sub.i of the inverter 2 is made extremely low or extremely large. If the operation frequency F.sub.i is made lower, however, the time quadrature of the voltage applied to the transformer 3 becomes larger and therefore it is necessary to make the sectional area of the core of the transformer 3 larger, resulting in increase in size of the transformer 3. Further, there is a limit in making the size of the transformer 3 and the operation frequency F.sub.i cannot be made so low to thereby cause a limit in the output voltage control range. Further, even if the operation frequency F.sub.i of the inverter 2 is made higher than an audio frequency so as to make noises low, the operation frequency F.sub.i may reach the audio frequency range to allow noises to become high when the operation frequency F.sub.i is made low in order to make the output voltage low in the case of a light load. Also in this case, accordingly, the operation frequency F.sub.i cannot be made so low to thereby cause a limit in the output voltage control range.